Elimination of spurious additions to radio signals



March 3, v1942. DE WITT R. GODDARD 2,274,829

ELIMINATION OF' SPURIOUS ADDITIONS 'IO RADIO SIGNALS Filed April 16, 1940 3 Sheets-Sheet l AATTORNEY.

March 3, 1942. 4|315 WITT R. GODDARD' 2,274,829

ELIMINATION OF SPURIOUS ADDTIONS TO RADIO SIGNALS INVENTOR. v DEN/77'?. GDDQD ATTORNEY.

March 3, 1942. DE WITT R. GODDARD 2,274,829

ELIMINATION OF SPURIOUS ADDITIONS TO RADIO SIGNALS Filed April 16; 1940 3 Sheets-Sheet 5 t INVENTOR. Ea/gag V Dfw/rr A. Gamm/2D Mud/Lf ATTORNEY.

Patented Mar.3,'1 942 ELIMINATION or sPUnroUs ADDITIONS 'ro Ramo SIGNALs.

.De Witt Rugg Goddard, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware ,l

'Application April 16, 1940, Serial 3 claims. (c1. 17a-6.7)

`This invention relates to facsimile and radio picture transmission and particularly to such transmission using the well-known constant frequency variable'dot signals such as disclosed in the patent of Shore and Whitaker 2,083,245, June 8, 1937. y

The constant frequency variable dot signaling system has various advantages, but it has one handicap due to multipath propagation in the upper atmosphere. It is well known that in radic transmission, signals take various paths between the transmitter and the receiver, some being refracted or reflected from higher ironized regions i of the upper atmosphere than others, consequently the signal first to arrive over the shortest path will have spurious additions made vto it by signals arriving over the longer paths.

The spurious additions to the signals make itl inecessary to reduce to speed of transmission so that this character elongation may be reduced to a small percentage of the signal character itself. In my invention I reduce the character distortion and practically eliminate it, so that the speed of transmission of pictures and facsimile is greatly increased.

One object of the invention is to increase the speed of picture and facsimile transmission by eliminating spurious additions to-the signals due to multipath and other causes.

Another object of the invention is to cause all signal characters at the transmitter to end at the same phase relation, so that at the receiver av counter-effect may be produced to block out spurious additions tothe end of the signals due to multipath eects.

Another object of' the invention is to utilize at the transmitter a saw-'tooth screen frequency that causes the variable dots or signals to end in the same phase relation with each other.

Another object of the invention* is to use at the receiver a saw-tooth generator voltage reversed in respect to the saw-tooth voltage at the transmitter, so that a counter-voltage may be produced to eliminate spurious additions.

Other objects' of the invention will appear in the following description, reference being had to the drawings in which:

Fig. 1 is an illustration of the transmitting system, well-understood units being indicated in block diagram.

Fig. 2 is an illustration of the receiving arrangement, well-understood parts also being shown in block diagram.

Fig. 3 is a graph of the saw-tooth transmitter voltage and a-graph of the scanner' output.

Fig. 4 is a graph of the combined output ofthe scanner and the saw-tooth generator.

Fig. 5 is a graph of the output of the square wave amplier of Fig. 1.

Fig. 6 is a graph of the radio signals tra'nsmitted by the apparatus in Fig. 1.

Fig. 7 is a graph of the radio signals received at the receiving station with spurious additions.

Fig. 8 is a graph of the reversed saw-tooth generator voltage at the receiver.

Referring to Fig. 1, the scanning apparatus generally indicated at I -consists of a drum 2 rotated by a synchronous motor 3 fed from a standard frequency unit 4. There are a number of standard frequency units well known in the art, one being a tuning forkunit such as described on pages 76 to 'I9 of an article by Callahan, Whitaker and Shore, in a book entitled Radio Facsimile, volume 1, and published by RCA Institutes Technical Press, New York, N. Y. Such a standard frequency unit is capable of maintaining a constant frequency having a maximum-variation of only one part in a hundred This contains a photoelectric cell 8 for converting variations in light into variations in electrical potential. The variations of potential are produced in`resistor 9 by a battery I0 having a potentiometer Il to which one electrode of `the photo-cell, say the anode, is adjustably connected. In this case the cathode is connected to the upper end of the resistance 9, as shown in the'drawings, and the positive terminal of the potentiometer is connected to the lower end of that resistance. Amplifier tube I2 has its grid connected to the negative end of resistance 9 and its cathode is connected to the other end of the resistance through a negative bias source Il. The plate or anode-of the tube I2 is connected to the 'positive terminal of a source of supply through the primary of the transformer I4, the

negative terminal of this source ofsupply being connected to ground and to the cathode.

With the arrangement described, total black 1 `picturewill produce minimum current or no current at all. This is the customary way of confnecting .the scanner, but of course this may be f reversed and maximum anode current produced by the white portions of the picture by reversing the input connection of amplifier tube I2 so that it works up from cut-ofi' instead of down to cutoff, as shown' in Fig. 1. The output of transformer I4 is fed into a suflicient number of amplifying stages l5 to produce the desired arnplification-of the output of the picture scanner. These amplifying stages are well known in the art and are indicated by blocks diagram only.

The output of the amplifying stages I5 is rectified at I6, preferably by Fleming tube rectiers, but this has been indicated generally, as the particular form of rectification per se forms lno part of my invention. The output voltage of the rectifier I6 in resistance I1 for a' brief instant may be indicated by the graph I8 in Fig. 3. This is to be combined with the output of the saw-tooth generator generally indicated at I9 in Fig. 1.

The saw-tooth generator I9 is well known in` the art and various circuits and tubes are used for producing saw-tooth voltages such as in sweep circuits of television'apparatus and the diagramlillustrated in Fig. 1 is by way of example only.

pThesaw-tooth generator I9 consists of a gas trriode 20 having its plate or anode connected to the positive terminal of a source -of supply througha suitable resistance 2 I. Resistances 22, 23 and' 24 are connected across from the negative terminal of resistance 2| to ground, which, as usual, is the negative terminal' of the source of supply. The grid 25 of the gas tube is connected through resistance 26 to the point 21 at the lower end of resistance 26. The cathode 28 of the gas tube is connected to ground through a pentode 29 and an adjustable resistance 30. The anode of the pentode 29 is directly connected to the cathode 28 and its cathode is connected to the upper end of resistance 30, as shown in Fig. 1. One grid 3| of the pentode is also connected to this end of the resistance. grid 32 is connected directly to ground and a third grid 33 is connected to the point 21.

A lcondenser 34 is connected between the anode and cathode of the gas tube 20 and the charging and discharging of this condenser in connection with the gas tube is whatproduces the sawtooth voltage. The pentode 29 in this hook-up has-no function other than to keep the charging current of the condenser 34 substantially constant. and a suitable inductance or other equivalent may be substituted for it, but the pentode is l preferred.

The output or load resistor 35 of the saw-tooth generator is tapped across the anode and cathode of the pentode through the adjustable resistance 30 and a blocking condenser 36, the latter being used to prevent passage of direct/current to the load resistor. The saw-tooth voltage produced at the terminals of condenser. 34, as will laterbe described, takes the form shown by graph 31 inIig. 3 and since this is the form of saw-toothwave that is desired for combination with thescanner output, the terminals of the condenser could be connected to resistance I1 if desired, but in that case it would be advisable to place the pentode between the positive terminal of the source of supply and the gas triode, as vshown in the sweep circuit of the Tube Handbook of RCA Manufacturing Company given for gas triode No. 88,5. l However, the reversal of this voltage in the illustration givenin Fig. 1 is read- A secondA load resistor 35 is the inverse of that shown by graph 31, because the drop in resistance 35 is -t-he plate voltage minus the voltage at the terminals` of condenser 34. It is well known that the anode current in an amplifying stage such as indicated at 38 is in phase with its input voltage; therefore the anode current in this stage also is the inverse of graph 31. The output voltage of this stage 33 between terminal 40 and ground terminal 4I is the supply voltage minus the drop in resistance 42 and therefore this output voltage is the inverse of the input voltage, that is, the output voltage is shown by graph 31.

Resistance 42 is adjustable and an adjustable resistance 43 is also .shown in thecathode lead. This adjustability taken withterminals 44 permits one to make proper connections and 'adjustments to produce a saw-tooth voltage 'that is the inverse ofgraph 31 for the saw-tooth generator at the receiver. The graph of receiver saw-tooth voltage is shown at.45 of Fig; 8, though of course this reversed saw-tooth voltage could be obtained by connecting directly to the load resistance 35 t and in various other ways.

As the circuit is connected in Fig. 1, the sawtooth voltage and the amplified output of the scanner are combined in -the input circuit of tube 46, having negative bias 46'. The output of this tube 46 is fed into a square wave amplifier 41 and this is fed into the transmittingapparatus 48 and the scanner is radiated for transmission t`o the receiver. Square wave amplifiers, modulators and other apparatus for transmission of signals are well known in the art and the particular formused is immaterial as far as my improvement vis concerned. This apparatus' has therefore been indicated in block diagram'only to prevent needless complication of this disclosure.

At the receiver any suitable receiving apparatus 49 may be used, consisting of radio frequency amplifiers, heterodyne oscillators, detectors and audio amplifiers, as Well as tone frequency generators and rectifiers., These are all well known in the art and per se form no part of my invention; therefore to simplify the disclosure th'ese have been indicated only generally by the block diagram 49. The operation of the entire system will be later described, but in brief it may be said that the signal output of the apparatus 49 takes the form of the graph in Fig. 7.

This signal is passed through a chopping tube 53 for elimination of the spurious tails or additions Aand'then passes into the illuminating orother element 5I for producing a record on the drum 52 rotated in synchronism with the drum 2 at the transmitter by a synchronous motor 53 fed from a standard frequency unit 54 through a suitable automatic phasing unit 55. This standard frequency unit is the same as the standard frequency unit 4 and will remain in synchronism with it to one part in about a hundred thousand.

However, slight corrections of the speed of the l drum 52 .are produced by the phasing Yunit 55 consisting of well-known capacity and inductance ture is connected -to the negative terminal and to the switch contacts 6I so that this terminal of the armature is always connected by the switch to the positive terminal.

Recorder drum 52 has a lug 52 engaging a switch blade 63 once each revolution. When the switch blade is operated by the lug, contacts 84 are closed and magnet 85 is energized by current from source of supply 66. When .the projection 82 disengages the switch lever 83, this lever springs back into normal position, opens contact 84 and closes contacts 81, which short-circuits the magnet 88. Magnet 68 is energized by the signal .pulses from the receiver 49 after appropriate amplication and rectication at 68.

The input circuit of tube'` 50 is connected to the standard frequency unit, causes the condenser always to strike in a definite phase relation'to the standard frequency. The frequency of charge and discharge of the condenser may be the same as that of the standard frequency unit v or it may be any multiple thereof, depending potentiometer `1li in the output of the reversed I saw-tooth generator 1I. As already stated, this generator may be the same as that indicated at I8 and 38 in Fig. 1, except that the connections to the 38 amplifier are so made that the voltage is the inverse of that shown in Fig. 3; that is, it is like graph 45 in Fig. 8.

The saw-tooth generator 1I in Fig. 2 is synchronized with the local standard frequency unit 54 by connection thereto through .phasingunit 55. The connections from the saw-tooth generator 1I are passed through an additional phasing unit 12 so that hand adjustments may be made to alter the phase of the saw-tooth voltage and thus place it in the right time-relation to place the peaks 13 (Fig. 8) in opposed phase with the peaks of the saw-tooth generator in Fig. 3. The bias 15 in chopper'tube 50.0f Fig. 2 is so proportioned that the tube conducts current, being above the cut-o point, except when the peaks 13 of the saw-tooth voltage above a value indicated by the dotted line 16 send the input voltage below the cut-olf point and thus block the tube. The operation of the system is as follows: The scanner sends out signal pulses varying from zero for pure white to maximum for black,

though as previously said, the opposite could be the case, if desired, by reversing the connection toresistance 8 or otherwise. The output is appropriately amplified, preferably at audio frequency, at I5 as distinguished from direct current amplication. The audio output .of the amplier I5 is then rectied at I6, which produces a potential in resistance I1 that for a brief instant may be depicted as an amplitude variation such as graph I8 in Fig. 3. This indicates, with the assumptionmade, that the scanner is moving from lighterv to darker elements in the subject being scanned.

To convert this amplitude variation into constant frequency `variable dot signals, the voltage in resistance I1 is combined with the saw-tooth voltage at the output of amplifier 38. While the operation of a saw-tooth generator is well known in the art, it will be advantageous to digress to briefly disclose how the saw-tooth voltage is' produced. 4

The voltage applied to condenser 34 charges it in accordance with the well-known voltagetime characteristic, which may be indicated by substantially a straight line such as the`slanting portion 11 of graph 31 Auntil the voltage' at the time t is suiiicient to cause the gas triode 20 to strike` with the bias produced across resistance 24 plus the voltage at that instant introduced into resistance 28 by the standard frequency, which upon the time constants of the charging circuit.

The condenser discharges practically instantaneously through the gas triode, which produces sistance 35 therefore varies inversely as the voltage of the condenser. That is; the voltage will take the form of the saw-tooth graph of Fig. 8, though this graph is not intended to show the relative values of the voltage.

The amplifier 38 as connected with the adjustment indicated in full lines, will reverse this voltage between th'e terminal 40 and ground, as previously explained, so that a voltage of the form. of 'graph 31 is combined with the scanner output as at I8 (Fig. 3) in the input of mixing amplifier tube 48. The negative bias 48 of the grid of tube 46 is adjusted vto block the tube with zero scanner output. Hence the saw-tooth voltage can pass only as th'eI scanner output rises above zero. The output current in tube `46 therefore takes the form shown in graph of Fig. 4. The output of tube 46 is passed through the square wave amplifier, which may be adjusted to have no increase of output above a voltage limit indicated l by the dotted line 8l. This output will take the general form of graph 82 in Fig. 5.

An inspection of Fig. 5 shows that the amplitude variation of the scanner has been converted to constant frequency variable dot length variation. This frequency is the same as that of sawtooth generator I9 which is the same as, or is any desired multiple of, the standard frequency of unit 4.

The output of the square wave amplier may be further amplified either with or without tone frequency conversion and used to modulate an appropriate radio oscillator. All of this apparatus is Well known and is generally indicated at 48.

The radiation from the antenna will Vtake the form indicated in the graph of Fig. 6.

The Ysignals received and converted by the ree ceiving apparatus 48 of Fig. 2 will usually not have the form of graph 82 oi' Fig. 5, due to multipath eiects. As already stated, they will have tails or additions, as indicated at 84 in Fig. 7. The apparatus, the operation of which has thus far been described, is known in the art and is notpart of my invention except insofar as it may be a necessary part of the combination claimed.

The vsignals of Fig. 7 have to pass through chopper tube 50 before reaching the recording light or equivalent unit 5, and the passageis controlled by the variable bias applied to the grid of tube 58 by the potentiometer 18. The adjustment of the-circuit is such that the tail part of the signal is blocked out by the saw-tooth tip voltages in the potentiometer.

The saw-tooth generator 12 is like that of generator I8 but the voltage curve is the inverse of Fig. 3 or like that in Fig. 8. One way this reversal can be accomplished has already been d emay be 810 cycles per second, for example. Thus '75 scribed. The saw-tooth generator is adjusted t9 shift which is corrected by the phasing unit 55 and this is adjusted automatically by the motor 56.

If drum 52 is in synchronism with drum 2' at the transmitter, then magnet 65 is energized and maximum current from the transmitter also energizes magnet 66 because the impulses sent -out by the gripper 5 are caused by its dark surface. The magnetic pull is thus balanced and switch bar 6I is not operated. If drum 52 at the receiver creeps ahead of synchronism, switch 53 will close contacts 64 while the scanner at the transmitter is receiving light from the pure white edge of the paper just ahead of the gripper 5. No signal will therefore be transmitted and magnet 68 will not be energized. Magnet 65 will lthen close contacts 60 and the energized field will rotate motor 56 in the direction that retards the phase by operation of phase unit 55 and thus retard motor 53 drivingl drum 52. This brings the drum 52 into synchronism. If the drum 52 drops behind the transmitter drum 2, the sigv nals sent out by the dark surface of gripper 5 will energize magnet 68 before the switch 63 is operated by lug 62. This will close contacts 58 and energize the other field of motor 56' and the phase unit 55 will be rotated in the opposite direction to advance the phase of the driving voltage of motor 53. This will speed it up and bring drum 52 back into synchronism.

Since saw-tooth generator 1I takes the corrected phase produced by thephase unit 55, it will have its phase correctly adjusted. However, its phase will be initially adjusted by thehandoperated'unit12, so that it will be 180 out of phase with the saw-tooth voltage of the transmitter.

It will thus be seenthat the peaks 13 arer always in position to block the passage of the tails added to the signal by the multipath effects. The signals received by the lamp or other unit 5I of the receiver have the form of those transmitted at the receiver as shown by graph 82 of Fig. 5.

It will be clear that other blocking means may be provided in the place of tube 50, such tube for blocking out spurious additions to the' being given merely as-one example. Other means of correcting the phase relation between the receiver and transmitter may also be used. My

invention is therefore not to be limited to the embodimentdisclosed.

nal character voltages, each ending at the same phase of a predetermined frequency, means forr producing a radio frequency wave, means for modulating said wave by said signal voltages, means for transmitting the modulated wave over a radio link, means at the receiving end of said introducing the signal character voltages and said alternating voltage into the mixing circuit to block out spurious additions to the signal voltages after said phase.

2. In radio systems, a scanner for producing an output voltage with varying amplitude, a generator having a saw-tooth voltage wave form with one side substantially perpendicular to the wave axis, means for combining the voltage of the scanner and that of said generator to p roduce constant+frequency variable dot-length signals ending at the same phase in the cycle, means'for transmitting said'signals, means for receiving and detecting said signals, a second generator having a saw-tooth voltage wave form with one side substantially` perpendicular to the wave axis at said phase, and means for combining the voltage of the detected signals with the peaks of the voltage Wave ofthe second generator to block out spurious additions to the sigsignal character voltages, each ending at the same phase of a'predetermined frequency, means for transmitting the -jsignal characters over a.

radio link, means for receiving and detecting the signal character voltages at the receiving end of said link, means for producing an alternating voltage of said frequency having maximum value at said phase, a mixing circuit, means for intro,-

ducing the signal character voltages and said alternating voltage into the mixing circuit to block out spurious additions to the signal voltages after said phase,l means fortransmitting said signals, means .for receiving and detecting the` signals, a second generator having a saw-tooth voltage wave of the same form as that at-the transmitter but 'out of phase therewith, and means to combine the detected signal-voltages with the voltage of the last-mentioned generator .sis-

nal characters after said phase.

DE Wrrr RUGG GODDARD.

Having described my invention, what I claimt Il. In radio signaling, means for producing 

